The present invention relates generally to semiconductor packaging technology and the manufacture of package assemblies. The present invention has particular applicability to quantitative methods of detecting residual flux in assembled package.
Integrated circuit devices are typically electronically packaged by mounting one or more integrated circuit (IC) chips or dies on a substrate, sometimes referred to as a carrier. In a flip chip assembly or package, the die is xe2x80x9cbumpedxe2x80x9d with solder to form a plurality of discrete solder balls over metal contacts on the surface of the die. The chip is then turned upside down or xe2x80x9cflippedxe2x80x9d so that the device side or face of the IC die can be mounted to a substrate having a corresponding array of metal contacts. Typically, the metal contacts of the substrate are coated with a solder alloy. Electrical interconnection of the die to the substrate is conventionally performed by aligning the die and substrate and reflowing the solder on the die and/or the substrate to electrically and mechanically join the parts. Directly coupling the die immediately below the substrate allows for an increased number of interconnections and improves voltage noise margins and signal speed.
Typically a flux composition is applied to either the die or the substrate to facilitate the formation of the interconnect. Flux acts as an adhesive to hold the components in place pending soldering and further minimizes metallic oxidation that occurs at soldering temperatures thereby improving the electrical and mechanical interconnection and reliability. Soldering fluxes fall into three broad categories: rosin fluxes, water-soluble fluxes, and no-clean fluxes. Rosin fluxes, which have a relatively long history of use and are still widely used in the electronics industry, are generally only moderately corrosive. Water-soluble fluxes, which are a more recent development and which are increasingly used in consumer electronics, are highly corrosive materials. No-clean fluxes, a very recent development, reportedly do not require removal from the circuit assemblies.
It has been observed that controlling the amount of applied flux is important irrespective of the type of flux employed in a particular packaging process, since enough flux must be used to effect a reliable metallurgical bond to electrically and mechanically interconnect the components. However, residues of any flux are believed to cause circuit failure if residual traces of the material are not carefully removed following soldering and, thus, remain on the electronic circuit assembly. Excess flux adversely impacts further circuit board manufacturing processes as, for example, delamination of underfill, leading to circuit failure. Particularly problematic are fluxes having corrosive constituents which, unless removed, will lead to corrosion of the soldered parts. Moreover, certain circuit board assemblers even require the removal of no-clean flux residues from circuit boards. Hence, determinations of flux residue following soldering is important in the fabrication of electronic packages.
Several methods of determining flux residue are known. For example, U.S. Pat. No. 4,530,601 to Tasset discloses a test method for determining residual rosin on cleaned circuit boards by washing the circuit board, adjusting the pH of the washings and then measuring the turbidity of the washings. U.S. Pat. No. 4,731,154 to Hausman et al. discloses a method for determining the organic contaminates remaining on cleaned circuit boards by spectrophotometric. Although the above-described methods provide determinations of flux residues, it is continually desirable to provide improved methods for determining flux residues from assembled electronic circuit packages. In particular, it is desirable to provide convenient and quick methods that can be preformed during high volume production of electronic circuit assembly.
Accordingly, a continual need exists for improved processes and/or assemblies for determining residual flux of electronic packaged components.
An advantage of the present invention is a convenient, precise and reliable method for determining flux residue.
Additional advantages and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the invention. The advantages of the invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by a process of determining residual flux residue on a device/substrate assembly. The method comprises: contacting the assembly with a solvent adapted to substantially remove flux residue from the assembly; and measuring the conductivity of the solvent to determine the presence of flux residue in the solvent obtained from the assembly. By measuring the conductivity of the solvent to determine flux residue, the present invention advantageously provides a convenient and quick process for determining the amount of residual flux remaining on a packaged assembly that can be practiced during high volume production thereby improving quality control.
Embodiments of the present invention include contacting the assembly with at least one solvent selected from the group consisting of an aromatic, alcohol, ketone, nitrile, amide, amine solvent or mixtures thereof by immersing and withdrawing the assembly in a fixed amount of the solvent and measuring the conductivity of the solvent with a volt meter to determine flux residue as low as 1-5 micrograms per mm2 of the assembly. Precise measurements of flux residue can be attained for any given package by preparing a series of standard solutions having a known amount of flux contained therein to provide a calibration curve relating the quantity of flux to conductivity of the solution and comparing the resultant conductivity of the contacted solvent to determine the amount of flux in the contacted solvent.
The present invention is applicable to the manufacture of packages comprising any device interconnected to a substrate in which flux has been used during the packaging process. In particular, the present invention is applicable to a process of forming a device/substrate assembly by mounting a device having a plurality of solder bumps to a substrate having a corresponding plurality of contact pads coated with flux and then reflowing the solder to form the assembly.
Additional advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawing and description are to be regarded as illustrative in nature, and not as restrictive.